1. Field of the Invention
The present invention relates to a packet communications apparatus and, more particularly, to a packet communications customer-station apparatus which is used for packet multiplexed communications such as an ATM (Asynchronous Transfer Mode) communications or the like.
2. Description of the Related Art
In recent years, the study of an access network employing the ATM technique is advancing rapidly.
FIG. 1 shows a configuration of an ATM-PON (ATM Passive Optical Network) as an example of an access network using an optical fiber as a transmission path. In the access network 1 in FIG. 1, an ONU (Optical Network Unit) 3 is provided as a customer-station in each subscriber home or office. An OLT (Optical Line Terminator) 5 is provided as an intra-network (intranet) apparatus in a station within the access network 1. An apparatus which serves as an interface of an access system called an OSU (Optical Subscriber Unit) 7 is provided in the OLT 5. The OSU 7 is connected to a star coupler (SC) 9 via an optical fiber 10. The SC 9 is connected to each ONU 3 in a plurality of subscriber homes via the optical fibers 10. For example, the OSU 7 and the SC 9 are connected by one optical fiber. The SC 9 and each ONU 3 are connected by one optical fiber. An upstream (direction from the ONU 3 to the OSU 7) ATM cell signal and a downstream (direction from the OSU 7 to the ONU 3) ATM cell signal are transmitted by a wavelength-multiplexed system using lightwaves of different wavelengths.
Each ONU 3 is constructed by, a common portion 11, a plurality of line cards (LC) 12, and a plurality of LC individual interfaces 13, and is connected to a plurality of terminals 15 by lines 16 via the LC 12. The terminal 15 is, for example, a personal computer, a server, a telephone, a communications apparatus, or a hub 17 to which these apparatuses are connected.
A section including the common portion 11 and the LC individual interfaces 13 of the ONU 3 faces the OSU 7 of the OLT 5 and serves as an interface of the access network 1. The common portion 11 controls the whole of the ONU 3.
User information from each terminal 15 is transmitted to the ONU 3 as ATM cell data or a data stream (i.e., data which is not converted to ATM cells). The LC 12 of the ONU 3 receives the data and performs, if necessary, an ATM cell conversion to the data stream which is not converted to the ATM cells. Between the LC 12 and LC individual interface 13, therefore, all of the user information data transmitted from each terminal 15 is converted to the ATM cells.
In response to the signal of the downstream direction, the LC 12 receives the ATM cells from the common portion 11, performs a dissolving process of the ATM cells if necessary in accordance with the connected terminals 15, and transmits the ATM cells or data stream to each of the terminals 15.
FIG. 2 schematically shows a configuration of the ONU 3. A line apparatus or line unit 20 including the LC 12 and the LC individual interface 13 corresponding to the LC 12 is provided for each line 16. The ATM cells in the upstream direction generated from each LC 12 are supplied to the LC individual interface 13 and stored in an input buffer 22 installed for each LC individual interface 13. A reading request of the ATM cell(s) stored in the input buffer 22 is transferred from each input buffer 22 to a priority control section 23.
In the priority control section 23, a reading order among the input buffers is determined in accordance with pre-assigned priorities and the ATM cells are read out from the input buffer 22 in order of the priorities. The ATM cells read out from the input buffers are multiplexed into one ATM cell stream in a multiplexing section 25. The resultant ATM cell stream is stored in a cell buffer 26.
A cell configuration of 56 bytes or 60 bytes has been proposed so far as an ATM cell format of the downstream signal, in which an overhead (hereinafter, referred to as PON-OH) to perform a control of ATM-PON is added to the ATM cell of 53 bytes. Instead of adding PON-OH, there is also another configuration in which a physical layer OAM cell (hereinafter, referred to as a PLOAM cell) as an OAM (Operation Administration and Maintenance) cell having ATM-PON control information or ONU control information is inserted in the ATM cell stream while the ATM cell is held to be 53 bytes. There is an access control information to control the access in the upstream direction as one of the ATM-PON control information that is carried by the PLOAM cell or PON-OH in the downstream direction. The access control information includes a transmission permission signal indicating which ONU 3 can output the ATM cell at the output time of each ATM cell in the upstream ATM cell stream of the access network.
Each ONU 3 outputs the upstream ATM cells only at the permitted time indicated by the access control information. The ATM cells are generated from a plurality of ONUs 3 at the same time as mentioned above, thereby preventing the ATM cells from colliding each other in the SC 9.
A transmission control section 28 of the ONU 3 extracts the transmission permission signal from the ATM cell stream of the downstream direction. When the transmission permission signal arrived to the ONU 3, the transmission control section 28 supplies a transmission command signal to the cell buffer 26. The ATM cells stored in the cell buffer 26 are read out in response to the command signal. In a case when a PLOAM cell in the upstream direction having response information to the control of the ONU 3 is generated in a PLOAM generating section 29, the PLOAM cell is selected preferentially by a selector (SEL) 30 and is output from the ONU 3. The reading operation from the cell buffer 26 is, therefore, be held until the next transmission permission is received.
Each PLOAM cell or ATM cell in the upstream direction is delayed in a delay circuit 32 by preset delay time for each ONU 3 in order to equalize the transmission delay time between the ONU 3 and the OSU 7 and is transmitted from the cell buffer 26 to the SEL 30. After that, a PON-OH is added to the transmitted cell in a PON-OH adding section 34. The resultant electric signal is converted to a light signal in an E/O converting section 35 and is output from the ONU 3.
In the standardization regarding the ATM-PON specifications, the access control method is also a focus of discussion. A method is proposed wherein a plurality of kinds of transmission permission are included in the access control information which is supplied to each ONU 3. That is, it is intended that the commands to designate various services can be included in the transmission permission, such as a command to designate one of the LCs 12, a command to designate user information such as image, audio, or data, or the like.
According to the conventional configuration shown in FIG. 1, however, the transmission control section 28 can control the cell buffer 26 only for one kind of transmission permission. Since the ATM cells stored in the cell buffer 26 have been multiplexed to one ATM cell stream, the ATM cell stream cannot be separated and it is impossible to cope with the plurality of kinds of service commands as mentioned above.
The present invention is made in consideration of the above problems and it is an object of the invention to provide a packet communications customer-station apparatus (hereinafter, simply referred to as a packet communications apparatus) which can generate a desired packet signal selectively in accordance with a kind of transmission permission received from an intranet apparatus.
To achieve the object, according to one aspect of the present invention, there is provided a packet communications apparatus which comprises a plurality of line units each of which is connected to at least one terminal apparatus and includes a packetizing circuit for packetizing a terminal information signal received from the terminal apparatus to generate a packet signal including a header information segment and a buffer circuit for storing the packet signal, a multiplexing circuit for multiplexing the packet signals stored in the buffer circuits of the plurality of line units and for generating a multiplexed signal, a transmitting circuit for transmitting the multiplexed signal to an intranet apparatus, and an access control circuit for receiving a transmission permission signal from the intranet apparatus and for generating a transmission command signal to instruct the transmission of the packet signals to the multiplexing circuit to each of the plurality of line units on the basis of the transmission permission signal.
According to another aspect of the present invention,
each of the plurality of line units includes a packet control circuit for receiving the transmission command signal, and
the packet control circuit includes a storage control circuit for storing each of the packet signals to the buffer circuit on the basis of the header information segment, and a transmission control circuit for controlling so as to transmit the packet signal from the buffer circuit to the multiplexing circuit on the basis of the received transmission command signal.
According to further aspect of the present invention,
the header information segment includes an identification information segment indicating an identification of a call,
the storage control circuit stores the packet signals including the same identification information segment into the buffer circuit as one packet signal group, and the transmission control circuit transmits the packet signals from the buffer circuit to the multiplexing circuit on the basis of the received transmission command signal and the identification information segment.